1. Field of the Invention
Embodiments of the present invention generally relate to the field of light-emitting diode (LED) technology and, more particularly, to a vertical light-emitting diode (VLED) structure.
2. Description of the Related Art
Luminous efficiency can be defined as the total apparent power of a light source to its actual total input power (luminous flux divided by input power). Having units of lumens per watt, luminous efficiency measures the fraction of power which is useful for lighting. As a type of light source, light-emitting diodes (LEDs) have been designed and developed over the past few decades to make improvements in luminous efficiency and increase the number of possible applications for these solid state devices.
Beginning with a conventional LED structure whose cross-section is shown in FIG. 1, one can see why the luminous efficiency of these devices is poor. A conventional LED 100 is formed on a substrate 102 such as sapphire, silicon carbide, silicon, germanium, ZnO or gallium arsenide depending on the composition of the LED layers to be deposited. In FIG. 1, the GaN LED 100 is grown on a sapphire substrate 102. An undoped layer 104 may be deposited on the substrate 102, and this layer 104 comprises undoped GaN in FIG. 1. An n-doped layer 106 is disposed above the substrate 102 (and the undoped layer 104 if it is present), and this layer 106 comprises n-doped GaN in the figure. A multiple quantum well (MQW) active layer 108 is deposited above the n-doped layer 106, and this is where photon generation occurs when the diode is properly biased. A p-doped layer 110 is grown above the active layer 108, and FIG. 1 depicts this layer 110 comprising p-doped GaN as an example. Since the conductivity of some p-doped layers 110 is very poor, a semi-transparent coating 112 may be applied above the p-doped layer 110 and may comprise Ni/Au or indium tin oxide (ITO). This coating 112 acts as a contact layer facilitating the dispersion of current. A p-electrode 114 for external connection is added above the p-doped layer 110 (or above the semi-transparent coating 112 if it was present). An n-electrode 116 is connected to the n-doped layer 106 for external connection.
For the conventional GaN LED 100 on a sapphire substrate 102, the p-GaN layer 110 may be kept as thin as possible for two reasons. First, the p-GaN layer 110 contains mid-gap states that may absorb the photons emitted from the active layer 108, thereby decreasing luminous efficiency with increased p-GaN layer thickness. Second, the growth of the p-GaN layer 110 occurs at a high temperature, which may harm the active layer 108 formed at a much lower temperature.
FIG. 2 illustrates possible paths of photons for the conventional LED structure of FIG. 1. Photons generated in the active layer 108 are radiated and travel through the different layers of the LED 100. Some photons travel through the p-doped layer 110 and are emitted as output light. The more of these photons that are emitted as output light for the same amount of applied power, the higher the luminous efficiency will be. As previously mentioned, however, the mid-gap states of the p-doped layer 110 may absorb many of the photons. Also, some of the photons travel through the p-doped layer 110 and are reflected back due to the two different indices of refraction at an interface between two disparate entities according to Snell's Law. These reflected photons are further absorbed or travel through the p-doped layer 110, the active layer 108 and the n-doped layer 106. Some of the photons generated in the active layer 108 travel directly to the n-doped layer 106 and on into the substrate 102 where they do not contribute to the emitted light whatsoever. The conventional LED 100 suffers from poor light extraction due to the relatively large quantity of photons that are absorbed or misdirected.
To improve upon some of the design limitations for luminous efficiency of conventional LEDs, the vertical light-emitting diode (VLED) structure was created. The VLED earned its name because the current flows vertically from p-electrode to n-electrode, and a typical VLED 300 is shown in FIG. 3. To create the VLED 300, an n-doped layer 302 is deposited on a substrate (not shown), and this may comprise n-GaN, as shown in the figure, or a combination of undoped GaN and n-GaN. A multiple quantum well (MQW) active layer 304 from which the photons are emitted is grown above the n-doped layer 302. A p-doped layer 306 is deposited above the active layer 304, and FIG. 3 depicts p-GaN as an example. This p-GaN layer 306 is kept as thin as possible to prevent harming the active layer 304 during deposition and to reduce absorption by the mid-gap states as mentioned above.
A reflective layer 308 may be formed above the p-doped layer 306, and then a thick conductive metal layer 310 is created above the reflective layer 308. In addition to allowing handling of the VLED devices after the removal of the initial substrates, the metal layer 310 dissipates heat more effectively than substrates, such as silicon and sapphire, of conventional LEDs. The reflective layer 308 is strategically placed to reflect errant photons back into the intended direction of light emission. The reflective layer 308 prevents photons from traveling beyond the semiconductor layers 302, 304, 306 and into the substrate for absorption. Heat dissipation away from the active layer 304 and reflection of wayward photons improve luminous efficiency.
Once the metal layer 310 is formed, a series of handling operations occurs that, in effect, flip the structure over and remove the VLED devices from the initial substrate (not shown) adjacent to the n-doped layer upon which the semiconductor layers 302, 304, 306 were deposited. For one external connection, either the metal layer 310 can act as a p-electrode, or as depicted in FIG. 3, an additional protective metal p-electrode 314 can be added to the bottom of the metal layer 310 if needed to improve the contact resistance and the p-electrode integrity. For the other external connection, an n-electrode 312 is added to the top of the n-doped layer 302. A semi-transparent coating is not necessary since the n-doped layer 302 has good conductivity and the p-doped layer 306 has the metal layer 310 and reflective layer 308 to spread out the current.
FIG. 4 illustrates possible paths of photons for the typical VLED 300 described above and shown in FIG. 3. Photons generated in the active layer 304 are radiated and travel through the different layers of the VLED 300. Some photons travel through the n-doped layer 302 and escape as output light. Those photons that did not escape get reflected by the surface of the n-doped layer 302 and travel back through the VLED layers 302, 304, 306. Once they hit the reflective layer 308, they are again reflected to travel back through the p-doped layer 306, the active layer 304 and the n-doped layer 302 before expectantly escaping out. Other photons emitted from the active layer 304 travel into the p-doped layer 306, get reflected by the reflective layer 308, and then travel back through the VLED layers 302, 304, 306 as described above. Moreover, some of the photons will be absorbed in the various layers 302, 304, 306.
Despite the fact that many of the photons in the typical VLED 300 are eventually routed in the intended direction of light emission due to the reflective layer, some of the photons are absorbed and reduce the overall light output. Accordingly, what is needed is a VLED structure that maximizes the number of photons that are generated and that escape out of the n-doped layer to improve luminous efficiency.